Fluoroscopy is a process for obtaining continuous, real-time images of an interior area of a patient via the application and detection of penetrating x-rays. Put simply, x-rays are transmitted through the patient and converted into visible spectrum light by some sort of conversion mechanism (e.g., x-ray-to-light conversion screen and/or x-ray image intensifier). Subsequently, the visible light is captured by a video camera system (or similar device) and displayed on a monitor for use by a medical professional. Typically, this is done to examine some sort of ongoing biological process in the human body, e.g., the functioning of the lower digestive tract.
Currently, most fluoroscopy is done using x-ray image intensifiers. These are large, vacuum tube devices (i.e., akin to a CRT or conventional television) that typically receive the x-rays in an input end, convert the x-rays to light and then electron beams, guide, accelerate, and amplify the electron beams via internal electrodes, and convert the electron beams to a minified visible image at the device's output end. An example of an x-ray image intensifier is shown in U.S. Pat. No. 5,773,923 to Tamagawa (see FIGS. 1 and 2 and accompanying description).
Typically, directly viewing the output end of an x-ray image intensifier is difficult and/or undesirable. For example, x-ray image intensifiers usually have to be positioned where direct viewing of the intensifier output is physically difficult (e.g., on a positioning gantry above a patient). Furthermore, tube outputs are often times not optimized for direct viewing, and it may be necessary to record or process the image for enhanced viewing, archival purposes, or for image analysis. Accordingly, image intensifiers are oftentimes used in conjunction with television cameras with computer imaging systems that transfer the tube image to a television or computer monitor. An example of such a system is shown in U.S. Pat. No. 4,503,460 to Sklebitz.
While x-ray image intensifiers (alone or as part of a larger system) are well established and have proven very useful over the years, they have certain drawbacks. One of these is that the image intensifier tubes are necessarily quite bulky and heavy, e.g., like a television CRT. Accordingly, they are ill suited for applications where space and/or weight are an issue. Additionally, x-ray image intensifiers suffer from inherent temporal and spatial instability, poor contrast ratios, and pincushion distortions.
Numerous recent attempts have been made to replace vacuum tube image intensifiers with solid-state, “flat panel” x-ray detectors for “directly” converting x-rays into electronic signals suitable for computer processing and display. Such detectors usually comprise some sort of large-area, composite, semiconductor-based panel configured to detect x-rays that hit the panel. For example, a typical flat panel detector might include a layer of photoconductor for converting incident x-rays into electrical charge, and a very large area integrated circuit, itself comprising an array of field effect transistors interconnected with thin film transistor switches, for readout of the charge. Systems based on optical photodiodes in an array in contact with a scintillator are also known.
While such detectors are thin and easy to hold and position, they are not well suited for fluoroscopy. This is because they have an almost irresolvable noise problem at the very low x-ray dose rates required for fluoroscopy. More specifically, because fluoroscopy involves examining an ongoing biological process (as opposed to taking an x-ray “snapshot”), patients are subjected to an ongoing dosage which must be minimized. As such, only a small amount of x-rays can be applied, or patients would quickly be injured. This is a problem for existing flat panel detectors, because the millions of signal lines used to read the x-ray induced charge (i.e., to scan the pixels) are as long as the panel itself, which is typically 25–50 cm (10–20 in) long to provide a large enough viewing area (i.e., the panel has to have a large enough area to facilitate viewing of the area(s) of interest—typically, the same or similar size as conventional x-ray film). As such, these long lines have significant electrical capacity (on the order of picofarads) to other lines and to ground. This limits the detectable signal, as electrons forming the signal current will have to charge and discharge this parasitic capacitance. When the signal is only a few hundred electrons (as may be the case with low levels of x-rays), the voltage changes in the signal lines may only be in the order of microvolts, which are too low to be seen above the inherent input device thermal noise exhibited by all amplifiers. This inherent electrical noise gets worse with high speed readout (higher bandwidth), which is required for 30 frames-per-second (“fps”) fluoroscopy. For example, for low numbers of x-ray photons (e.g., 10) per detector pixel, as needed for fluoroscopy, each x-ray photon results in a signal per pixel (as finally read out in a flat panel device) of around 600 electrons, meaning that image data is lost in the typically high noise levels of 2000–8000 electrons of the readout line amplifiers. Furthermore, because the manufacturing yields of such large-area (e.g., 900 cm2) transistor arrays are typically quite small, existing flat panel detectors can be very expensive. Subdividing the panel readout may help, but that quadruples the already large number (1000's) of output amplifiers, since one is associated with each readout line.
Other x-ray detectors utilize optical systems and associated components as an alternative to oversized semiconductor panels. For example, in U.S. Pat. No. 5,723,865 to Trissel et al. (“Trissel”), a CCD camera (CCD sensor coupled to a lens system) is focused on the output surface of a special, composite x-ray conversion or scintillation screen, at the heart of which is a flat, single crystal of cesium iodide (CsI). In use, x-rays fall on the scintillation screen, and are converted into visible light, which is then picked up by the camera. However, because a single-crystal CsI screen and single CCD camera are used, the system in Trissel is only suitable for viewing small areas, e.g., as in a mammogram, its intended purpose.
U.S. Pat. No. 5,412,705 to Snoeren et al. (“Snoeren”) shows another x-ray imaging device. Here, x-rays are applied to an object under examination, and are then converted into visible light by way of a special screen. The light from the screen is then focused by a lens system/array, the output of which is directed to an array of CCD (charge-coupled device) image sensors, which is operably connected to a computer or other electronic processing equipment for image reconstruction and/or display. This provides a wide viewing area without a large-area semiconductor panel. However, the CCD sensors in Snoeren utilize weak-avalanche photodiodes for detecting incident light. These provide signal gain at each pixel, but also have disadvantageously-increased noise levels because of random fluctuations in the avalanche process, meaning that the detector is not well suited for very low levels of x-rays. Additionally, the lens system in Snoeren is not claimed as optimized for use with the low light levels associated with x-ray quantum limited imaging. Rather, Snoeren tries to improve the situation by causing more of the emitted light to be aimed toward the CCD array by a complex fluorescent screen fabrication using special optical techniques.
Accordingly, a primary object of the present invention is to provide a flat-panel x-ray detector or imager, suitable for use in fluoroscopy and other very low x-ray dosage applications, and that does not require the use of large-area semiconductor or integrated circuit arrays and associated long signal lines.
Another object of the present invention is to provide a system where every single, individual x-ray photon that interacts in the input screen produces an unambiguous signal in the output voltage. This is called “quantum limited” imaging, where x-ray statistics alone determine the noise level in the final image.